In bitumen froth treatment processes, solvent or diluent is added to a bitumen froth to separate a diluted bitumen stream for further processing. In a paraffinic bitumen froth treatment process, for example, bitumen froth derived from oil sands is combined with paraffinic solvent and then supplied to a settling vessel in which a bitumen rich fraction is separated from a bottoms fraction rich in asphaltenes, water, solvent and solids as well as residual amounts of bitumen. This bottoms fraction is often referred to as solvent diluted tailings or froth treatment tailings.
Solvent diluted tailings are preferably treated to recuperate the paraffinic solvent, which is subject to environmental discharge regulations and a valuable commodity, prior to disposal of the resulting solvent recovered tailings containing primarily water and solids. Solvent diluted tailings may be treated in tailings solvent recovery units that include flash vessels.
Flash vessels conventionally used to recover diluent from froth treatment tailings are specified for a feed flow and feed temperature so that, at the stage column pressure with optional stripping, steam vaporizes the diluent for recovery in the overhead condensing system.
However, variations in feed flow and feed temperature result in several challenges that affect recovery and unit reliability. High feed flows can increase liquid loading on flash column internals which directionally increases the time required for diluent to separate. In addition, low feed flows can cause “short circuiting” in the flash column when feed does not adequately cover internals and tends to promote depositions of froth treatment tailings minerals, bitumen and asphaltenes on the surfaces of internals. Furthermore, high feed temperatures can increase the column pressure when water that flashes with diluent exceeds the condensing capacity of the overhead system. Also, high velocities in the column can encourage unwanted entrainment of particulates that foul the overhead system. Low feed temperatures may also reduce the flashing of diluent from froth treatment tailings. These problems and challenges may owe to a number of factors, including feed properties of the bitumen froth, operating conditions of the froth treatment settling vessels and other parameters of the froth treatment plant.
A conventional approach currently practiced includes the addition of water to froth treatment tailings to ensure minimum line velocities are maintained to suspend solids in froth treatment tailings and avoid line plugging. This water may comprise waste water from various sources without control on temperature. Variations in froth treatment tailings both from flow and temperature perspectives have been observed. However, the implications of adding water on performance of flash separation may include several drawbacks.
As more general background on paraffinic froth treatment (PFT), for which solvent diluted tailings are derived, extraction processes are used to liberate and separate bitumen from oil sand so the bitumen can be further processed. Numerous oil sand extraction processes have been developed and commercialized using water as a processing medium. One such water extraction process is the Clarke hot water extraction process, which recovers the bitumen product in the form of a bitumen froth stream. The bitumen froth stream produced by the Clarke hot water process contains water in the range of 20 to 45%, more typically 30% by weight and minerals from 5 to 25%, more typically 10% by weight which must be reduced to levels acceptable for downstream processes. At Clarke hot water process temperatures ranging from 40 to 80° C., bitumen in bitumen froth is both viscous and has a density similar to water. To permit separation by gravitational separation processes, commercial froth treatment processes involve the addition of a diluent to facilitate the separation of the diluted hydrocarbon phase from the water and minerals. Initial commercial froth treatment processes utilized a hydrocarbon diluent in the boiling range of 76-230° C. commonly referred to as a naphtha diluent in a two stage centrifuging separation process. Limited unit capacity, capital and operational costs associated with centrifuges promoted applying alternate separation equipment for processing diluted bitumen froth. In these processes, the diluent naphtha was blended with the bitumen froth at a weight ratio of diluent to bitumen (D/B) in the range of 0.3 to 1.0 and produced a diluted bitumen product with typically less than 4 weight percent water and 1 weight percent mineral which was suitable for dedicated bitumen upgrading processes. Generally, operating temperatures for these processes were specified such that diluted froth separation vessels were low pressure vessels with pressure ratings less than 105 kPag. Other froth separation processes using naphtha diluent involve operating temperatures that require froth separation vessels rated for pressures up to 5000 kPag. Using conventional vessel sizing methods, the cost of pressure vessels and associated systems designed for and operated at this high pressure limits the commercial viability of these processes.
Heavy oils such as bitumen are sometimes described in terms of relative solubility as comprising a pentane soluble fraction which, except for higher molecular weight and boiling point, resembles a distillate oil; a less soluble resin fraction; and a paraffinic insoluble asphaltene fraction characterized as high molecular weight organic compounds with sulphur, nitrogen, oxygen and metals that are often poisonous to catalysts used in heavy oil upgrading processes. Paraffinic hydrocarbons can precipitate asphaltenes from heavy oils to produce deasphalted heavy oil with contaminate levels acceptable for subsequent downstream upgrading processes. Contaminants tend to follow the asphaltenes when the asphaltenes are precipitated by paraffinic solvents having compositions from C3 to C10 when the heavy oil is diluted with 1 to 10 times the volume of solvent.
High water and mineral content distinguish bitumen froth from the heavy oil deasphalted in the above processes. Some early attempts to adapt deasphalting operations to processing bitumen from oil sands effected precipitation of essentially a mineral free, deasphalted product, the ability to vary the amount of asphaltene precipitated, and the enhancement of asphaltene precipitation by addition of water and chemical agents.
Recent investigations and developed techniques in treating bitumen froth with paraffinic use froth settling vessels (FSV) arranged in a counter-current flow configuration. In process configurations, counter-current flow refers to a processing scheme where a process medium is added to a stage in the process to extract a component in the feed to that stage, and the medium with the extracted component is blended into the feed of the preceding stage. Counter-current flow configurations are widely applied in process operations to achieve both product quality specifications and optimal recovery of a component with the number of stages dependent on the interaction between the desired component in the feed stream and the selected medium, and the efficiency of stage separations. In deasphalting operations processing heavy oil with low mineral solids, separation using counter-current flow can be achieved within a single separation vessel. However, rapidly setting mineral particles in bitumen froth preclude using a single separation vessel as this material tends to foul the internals of conventional deasphalting vessels.
A two stage paraffinic froth treatment process is disclosed in Canadian Patent No. 2,454,942. In a froth separation plant, bitumen froth is mixed with overflow product from the second stage settler such that the solvent to bitumen ratio in the diluted froth stream is above the threshold to precipitate asphaltenes from the bitumen froth. For paraffinic froth treatment processes with pentane as the paraffinic solvent, the threshold solvent to bitumen ratio as known in the art is about 1.2 which significantly increases the feed volume to the settler. The first stage settler separates the diluted froth into a high dilute bitumen stream comprising a partially to fully deasphalted diluted bitumen with a low water and mineral content, and an underflow stream containing the rejected asphaltenes, water, and minerals together with residual maltenes from the bitumen feed and solvent due to the stage efficiency. The first stage underflow stream is mixed with hot recycled solvent to form a diluted feed for the second stage settler. The second stage settler recovers residual maltenes and solvent to the overflow stream returned to the first stage vessel and froth separation tailings. It is important to recognize the different process functions of stages in a counter-current process configuration. In this case, the operation of first stage settler focuses on product quality and the second stage settler focuses on recovery of residual hydrocarbon from the underflow of the first stage settler.
The process may be operated at temperatures that require controlling the pressure in either settler stage to limit solvent vaporization. The concentration of solvent in diluted bitumen and temperature for a specific paraffinic solvent such as pentane determine the solubility and hence the rejection of asphaltenes. While low asphaltene rejection maximizes bitumen recovery, the asphaltene content may limit processing options in upgrading operations particularity those based on hydrogen addition.
Furthermore, froth treatment tailings from either naphthenic or paraffinic froth treatment process contain diluent associated with unrecovered bitumen and the cost and environmental impact preclude directly discharging froth treatment tailings to tailings ponds.
Various treatment and recovery schemes are disclosed in literature. In one process, froth treatment tailings from the froth treatment plant are introduced to a flash vessel with internal shed decks maintained at sub-atmospheric pressures. Steam is introduced below the internals and the major portion of the diluent vaporizes together with water. The flashed vapours are removed and cooled to condense diluent and water which separate by gravity settling. Non-condensed vent gases are withdrawn from the condenser to maintain the sub-atmospheric pressure. The flashed tailings are pumped from the flash vessel to tailings disposal.
While diluent recovery for this process, which is disclosed in Canadian Patent No. 1,027,501 (Simmer), has been identified at 86%, actual practise as disclosed in Canadian Patent No. 2,272,045 (Brown et al.) achieve recoveries of only 60 to 65% of the diluent. This lower recover has been attributed to flashing at the feed inlet inducing feed to bypass the shed decks negating addition of steam below the shed decks. A proposed solution identified by Brown et al. was to operate the flash vessel at near atmospheric pressure which permitted feed distribution over the shed decks and increasing the steam addition to maintain vessel temperature to about 100° C. which could increase naphtha diluent recovery to 80% and the process could be applied to paraffinic diluent operations.
Recent investigations into attaining diluent recoveries as disclosed in Brown et al. identified variations in froth treatment operations, processing froth treatment slops, addition of process water for flushing equipment and maintenance of minimum velocities in slurry lines, which result in feed to the tailings treatment flash column varying by about +/−25% with respect to flow and about +/−10% with respect to temperature. The variation in feed flows and temperatures translates to varying the enthalpy of the feed stream to the column.
The diluent recovery process disclosed by Simmer flashes feed to a flash temperature such that the enthalpy of vaporized flash components matches enthalpy released from the flash liquid and the flash temperature governs vapour pressures of vaporizing components. Given the relative volatility of diluent hydrocarbons, one can expect a direct relation between feed temperature, flash temperature and diluent recovery. However, the investigation identified that increased feed temperatures for the same feed flow did not proportionately translate to increased diluent recovery due to increased vaporization of water. Stable operation of the column in terms of flash temperature and pressure was found marginally below the boiling point of water for the operating pressure and with small increases in feed enthalpy resulting in upsets as the water can essentially boil.
The upsets affect the flash column in various ways, two of which are the following. Firstly, boiling on shed decks results in equipment damage to the extent that the shed decks experience frequent structural failure. Secondly, the vapour velocity in the column increases, for instance by an order of magnitude, exceeding design guidelines such a set out in “Design Two-Phase Separators within the Right Limits” W. Svrcek, et al. Chemical Engineering Progress, October 1993, which leads to problems related to entraining solids and bitumen into the overhead system.
In the overhead system, bitumen acts a binder for the solids to adhere on surfaces in the overhead system. The adherence of solids to components of the overhead system restricts vapour flow to the separator. The adherence of solids on condenser heat transfer surfaces reduces cooling and condensing of vapours which increases the non-condensed gases to be vented. Directionally, solids adhering on surfaces in the overhead system increases column pressure which reduces feed flashing and actual diluent recoveries, for instance to 60 to 65% over the operational cycle. The contribution of increased steam of Brown et al. to improve diluent recoveries due to the reduced partial pressure created by the superheated steam is largely offset by the increased water vapour reporting to an overhead system restricted by the adherence of solids. Solids adherence to surfaces increases the pressure drop between flash vessel and condensate drum; this, in turn, increases the flash pressure, which lowers the extent to which diluent flashes at higher pressure. Over the operating cycle, the deposits of solids cause column performance to deteriorate and regaining performance is achieved by shutting down the column and associated systems for repair and cleaning.
It is clear that the known techniques and methods of treating froth treatment tailings have several drawbacks and shortcomings.